The Federal Communications Commission (FCC) in United States requires all network providers to be able to determine all the mobile users locations to within an accuracy of 100 meters in 67% of call measurements for network based systems. The positioning capability is 3GPP system also has become one of the requirements. This highlights the importance of the need for an accurate positioning method within mobile systems.
The positioning of the mobile user could provide a wide range of services such emergency rescues, location based service, location-sensitive billing, fraud detection, cellular system design and resource management and also intelligent transportation systems. The cellular geo-locating of a mobile station (MS) uses the existing well established infrastructure of cellular base stations (BTS) to geo-locate the MSs as they transmit over standard cellular frequencies. The advantages of using cellular positioning system over GPS is less complexity in the handset (no need for GPS transmission) and therefore lower cost. Currently, there are several techniques that could be used in mobile positioning namely, the Time Of Arrival (TOA), Time Difference Of Arrival (TDOA), Angle Of Arrival (AOA) and also hybrid techniques. All these techniques use signal strength to determine distance between the MS and BTS. The TOA, TDOA and signal strength uses range prediction to determine the distance of a MS away from the BTS, and AOA uses the direction of arrival of signals to predetermine the angular displacement of the MS from the BTS. The above-mentioned techniques except AOA need at least three BTS to geo-locate the MS, i.e. tri-circular algorithm. This is shown schematically in FIG. 1.
The Observe Time Difference (OTD) and Timing Advance (TA) methods proposed for GSM uses the timing advance measurements to determine the distances between the MS and the BTS, i.e. at least three BTS are involved to estimate the user's position. The TA mechanism has not been foreseen as a means to estimate the exact range of the MS from the BTS but to ensure that the MS data reaches the BTS at its correct time slot (TA gives the round trip propagation delay time of the signal). This method therefore, suffers from one bit resolution error. In GSM 1 bit is equivalent to 3.6923 μs. This corresponds to ±553.845 m. Hence, even under ideal conditions, i.e. line of sight, no multi-path and no multiple access interference, a positioning method based on TA and Triangle BSS will still have an inaccuracy error of approximately ±553 meters (1106 m uncertainty).